Storm worm smackdown as researchers unpick control system

But legal fears may kibosh clean-up

A team of security researchers have developed a technique for automatically purging the remnants of the Storm worm infection from the internet. But the approach - which involves turning the botnet's command and control system against itself - could run foul of computer hacking laws in Germany and elsewhere, which ban the modification of computer systems without consent.

Nonetheless, the work of the team from Bonn University and RWTH Aachen University have advanced knowledge about how botnets (networks of compromised zombie PCs) are established and maintained that could advance the development of more acceptable tracking and take-down techniques.

The analysis of the infamous Storm worm botnet by Georg Wicherski, Tillmann Werner, Felix Leder, and Mark Schlösser established that the network of compromised machines established by the Storm worm is far less resilient than previously suspected, Heise Security reports.

PCs compromised by Trojan agents associated with the Storm worm turned compromised machines into zombie drones under the control of hackers. These hackers (or bot-herders) issue commands to compromised machines through control servers. If connection with a control server can't be established, then zombie clients use P2P techniques to locate other control servers.

Agents spreading the Storm worm first appeared two years ago, posing as information on the storms ravaging Europe at the time. In the months that followed, infectious agents adopted a variety of guises, such as greetings cards.

Microsoft's Malicious Software Removal Tool made big inroads into cleaning up infected systems. The steady stream of new email baits designed to recruit fresh marks into the Storm Worm botnet has dried up of late, another factor that means the number of clients in the Storm Worm networks has shrunk to a fraction of its previous high.

Despite this, as many as 100,000 compromised clients are thought to remain infected, way down on the millions infected by the Storm worm as its peak but still a scourge to internet hygiene.

The German researchers reckon that the Storm worm could have been dismantled at a far earlier stage. Instead of taking a black box approach of analysing the behaviour of compromised clients, the German academics took the Storm worm apart byte by byte - analysing the zombie client programs to work out what made it tick. The researchers paid particularly close attention to how compromised clients communicated with zombie control servers.

The boffins then developed their own client, capable of hooking into the Storm worm network, discovering in the process that servers have are not required to authenticate themselves to compromised clients. They also discovered that their eavesdropping client was treated as a trusted participant in the network, trusted to relay queries and command instructions.

These two factors made it possible for the researchers to divert compromised drones to dummy servers, under their control, from where infected machines might be instructed to download clean-up agents. The security researchers wrote a clean-up utility - delicately called Stormfucker - but never sent it into play because of concerns the action might be illegal or, more to the point, prompt lawsuits in the event something went wrong with the clean-up process.

The boffins also developed techniques to distribute the clean-up process so that it might be able to handle large loads and would be better at withstanding attempts to force it off the net by hackers. ®